Westernized countries are experiencing striking increases in the prevalence of asthma. The inflammatory airway response in asthma may be the result of immune cells that are dysregulated towards environmental factors. The long-term goal of this project is to better understand the pathophysiologic mechanisms of persistent and recurrent Th2-type airway inflammation in asthma patients. Exposure to the fungus, Alternaria, is implicated in the development and exacerbation of human asthma. After intranasal exposure to Alternaria, naive mice showed eosinophilic airway inflammation, enhanced Th2 sensitization to innocuous antigens, and airway hyperreactivity. Alternaria activated dendritic cells (DCs) in vitro; this activation induced DCs to express costimulatory molecules and certain cytokines, and it inhibited production by DCs of the IL-12 family molecules and IL-10. Bioactive enzymes from Alternaria and a novel category of innate immune receptor, protease-activated receptors (PARs), on DCs are likely responsible for these immunological effects. Thus, the overall hypothesis is that immunoactive enzymes derived from a clinically important fungus, Alternaria, activate DCs through PARs, resulting in the induction of strong Th2 adaptive immune responses in the airways. To test this hypothesis, we will determine the role(s) of PARs in the activation and the immunoregulatory functions of DCs and the role(s) of PARs in the recognition of Alternaria products by DCs in vitro (Aim 1). Using mouse airway sensitization models, we will investigate the role(s) of PAR2 expressed on DCs in the development of airway Th2 immunity in response to Alternaria in vivo (Aim 2). We will use both proteomics and functional genomics approaches to identify the Alternaria- derived immunostimulatory enzymes that are involved in the Th2-driving effects of Alternaria (Aim 3). The ability of microbial enzymes to influence DC activation will represent a novel pathway to explain how the innate immune system recognizes environmental signals and influences the adaptive immune response. Elucidation of the underlying mechanism of immune cell activation and dysregulated Th2 responses in asthma patients will lead to a better understanding of the pathophysiologic mechanisms of persistent and recurrent airway inflammation and to the development of more specific and effective therapies and prevention strategies. Project Narrative: Patients with asthma have persistent respiratory health problems. This project will investigate how common environmental fungi cause, prolong and intensify this disease.